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Phase diagrams with very flat LCST phase separation line for aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions are theoretically derived on the basis of sequential hydrogen bond formation between polymer chains and water molecules (cooperative hydration) and compared with experimental spinodal curves. The two-phase region systematically changes its shape with the cooperativity parameter σ, and the spinodals turned out to be almost independent of the polymer molecular weight for strongly cooperative hydration (small σ) as observed in PNIPAM solutions. Hydration takes place sharply within a very narrow temperature region. The number of hydrogen-bonded water molecules on a chain is calculated as a function of the temperature and the polymer concentration.

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We examine the influence of the macromolecule chain length on the cloud point temperature (T cp ) and the temperature of the coil-to-globule transition (T M ) in aqueous solutions of hydrophobically modified (HM) telechelic poly(N-isopropylacrylamides) (PNIPAM) ranging in concentration from 0.01 to 35 g L -1 (0.1-310 mmol of NIPAM L -1 ). The telechelic HM-PNIPAM samples with n-octadecyl termini were obtained by RAFT polymerization of NIPAM in dioxane in the presence of S-1-n-octadecyl-S′-(R,R′-dimethyl-R′′-N-n-octadecylacetamide)trithiocarbonate as a chain transfer agent. Their molar mass (M n ) ranged from 12 000 to 49 000 g mol -1 with a polydispersity index lower than 1.20. The cloud point temperatures, measured by monitoring the temperatureinduced changes in scattering intensity, decreased significantly with increasing polymer concentration, this effect being more pronounced with decreasing polymer molar mass. In contrast, the temperature of the PNIPAM chain coil-to-globule collapse (30 ( 1 °C) was only slightly affected by solution concentration and polymer molecular weight. These results are interpreted in terms of the coexistence of two phenomena: association of the n-octadecyl terminal groups and hydration of the PNIPAM chains.